The electrodeless lamps with which the present invention is concerned generally comprise a microwave cavity within which is mounted a lamp envelope which contains a plasma-forming medium. This medium is energized by microwaves, R.F., or other electromagnetic energy, thereby creating a plasma which emits radiation in the ultraviolet, visible, or infrared portion of the spectrum.
In a typical electrodeless lamp the electrical energy is coupled to the cavity and to the lamp with a constant electric field geometric orientation which results in hot zones within the lamp envelope volume, and therefore non-uniformity in the radiation emitted from various portions of the envelope and in wall temperatures. Non-uniform wall temperatures unduly restrict the power which can be applied to the lamp, and non-uniform light emission is undesirable for some applications.
The distribution of light intensity in an electrodeless, microwave driven lamp is a complex function of many variables including the electrical power, the plasma-forming constituents in the envelope, and the geometries of the microwave power feed, the microwave resonant cavity, and the bulb envelope. The non-uniform distribution of light can be compensated for in the design of reflectors in some instances, but it is not always feasible to solve the problem of non-uniform light emission in this manner, and improved methods of increasing the uniformity of the intensity of light which is emitted from an electrodeless lamp are desirable.
Electrodeless lamps transfer a great amount of heat energy to the envelope surface. Electrical power which is coupled to the plasma-forming medium and the plasma by microwaves and which is not radiated away to the environment is absorbed by the envelope through conduction, convection, and radiation. This thermal loading of the envelope, which as noted above is typically non-uniform, requires that the envelope be cooled to protect it from temperatures which would soften or even melt it.
As noted above, non-uniform wall temperatures are undesirable, and U.S. Pat. No. 4,485,332 addresses this aspect of the cooling problem and provides a cooling method in which a stream or streams of a cooling gas are directed against the surface, including the hot spots, of an envelope which is being rotated. A relatively low rotation rate, such as for example, a rotation rate of 300 RPM was able to produce substantially uniform temperatures at the points on the surface of the envelope within a plane which was perpendicular to the axis of rotation, i.e., along lines of constant latitude. Slow rotation rates were successful in making the temperature distribution symmetrical in azimuth around the rotation axis because the heat capacity of the envelope resulted in cooling times in the range of seconds, i.e., times which are greater than the rotation period. However, these low rotation rates did not eliminate the non-uniformities of temperatures on the surface of the envelope along lines of constant longitude, that is, along great circles which passes through the poles.
U.S. patent application Ser. No. 674,631 filed Nov. 26, 1984 by Ury, et al. for "Method and Apparatus for Cooling Electrodeless Lamps" also addresses the cooling problem and describes a method of cooling electrodeless lamps by directing a stream of cooling gas at the lamp envelope and providing relative rotation between the lamp envelope and the stream of cooling gas. The method of relative rotation described therein included rotating the streams of cooling gas about the envelope. Japanese Application No. 229730/83 which corresponds to U.S. patent application Ser. No. 674,631 has been laid open.